package ui.swing.common;

import javax.swing.SwingUtilities;
/**
 * This is the 3rd version of SwingWorker (also known as
 * SwingWorker 3), an abstract class that you subclass to
 * perform GUI-related work in a dedicated thread. For
 * instructions on using this class, see:
 *
 * http://java.sun.com/docs/books/tutorial/uiswing/misc/threads.html
 *
 * Note that the API changed slightly in the 3rd version:
 * You must now invoke start() on the SwingWorker after
 * creating it.
 */
public abstract class SwingWorker {

	// value produced by the worker thread
	// see getValue(), setValue()
	private Object value;
	
	/**
	 * 下面的静态局部类用于保持对并发的工作线程的引用
 	 * Class to maintain reference to current worker thread
	 * under separate synchronization control.
	 */
	private static class ThreadVar {
		private Thread thread;
		ThreadVar(Thread t) {
			thread = t;
		}
		synchronized Thread get() {
			return thread;
		}
		synchronized void clear() {
			thread = null;
		}
	}
	
	private ThreadVar threadVar;
	
	/**
	 * 取得工作线程产生的值，如果工作线程未被构造则返回null
	 * Get the value produced by the worker thread, or null if it
	 * hasn't been constructed yet.
	 */
	protected synchronized Object getValue() {
		return value;
	}
	
	/**
	 * 用于工作线程设置自己产生的值
	 * Set the value produced by worker thread
	 */
	private synchronized void setValue(Object x) {
		value = x;
	}
	
	/**
	 * Compute the value to be returned by the <code>get</code> method.
	 */
	public abstract Object construct();
	
	/**
	 * Called on the event dispatching thread (not on the worker thread)
	 * after the <code>construct</code> method has returned.
	 */
	public void finished() {
	}
	
	/**
	 * Return the value created by the <code>construct</code> method.
	 * Returns null if either the constructing thread or the current
	 * thread was interrupted before a value was produced.
	 * 
	 * Be careful about using <code>get</code>. Because it blocks, it 
	 * can cause deadlock. If necessary, you can interrupt the thread
	 * (causing get to return) by invoking <code>interrupt</code>
	 *
	 * @return the value created by the <code>construct</code> method
	 */
	public Object get() {
		while (true) {
			Thread t = threadVar.get();
			if (t == null) {
				return getValue();
			}
			try { // ? 反复调这个没有问题吗？
				// someThread.join(long)：当前线程进入阻塞（Blocked）状态，
				// 直到someThread运行结束/到时后，转到 就绪（Ready）状态??
				t.join();
			} catch (InterruptedException e) {
				Thread.currentThread().interrupt(); // propagate
				return null;
			}
		}
	}
	
	/**
	 * Start a thread that will call the <code>construct</code> method
	 * and then exit.
	 */
	public SwingWorker() {
		final Runnable doFinished = new Runnable() {
			public void run() {
				finished();
			}
		};
		
		Runnable doConstruct = new Runnable() {
			public void run() {
				try {
					setValue(construct());
				}
				finally {
					threadVar.clear();
				}
				SwingUtilities.invokeLater(doFinished);
			}
		};
		
		Thread t = new Thread(doConstruct);
		threadVar = new ThreadVar(t);
		
	}
	
	/**
	 * Start the worker thread.
	 */
	public void start() {
		Thread t = threadVar.get();
		if(t != null) {
			t.start();
		}
	}
	
	/**
	 * A new method that interrupts the worker thread. Call this method
	 * to force the worker to stop what it's doing.
	 */
	public void interrupt() {
		Thread t = threadVar.get();
		if(t != null) {
			t.interrupt();
		}
		threadVar.clear();
	}
}
